First of all, check out the article on how to use monitoring. Note that monitoring is currently supported only on Mac computers running Chrome and on iOS devices with the Soundtrap app. For more information about this, check out the article on browser support.
Still feel like you should be able to hear something? Check all your connections and the volume controls of your microphone and/or audio input device (audio interface)!
If that didn’t sort it out, make sure you select the right audio device in the sound setup.